Heat treatment of materials by an electric arc has a long history since the mid-19th century, when this phenomenon was discovered. The devices able to generate high temperatures up to several 10,000 K were developed.
Building on these applications the transferred electric arc started to be used in the field of welding and cutting, processes where materials also undergo intense melting and a partial vaporization. All these methods use processed material as one of the electrodes. Innovations in this area have been known since the first half of the 20th century. Their common drawback is that they use the welded or cut material/metal/as one of the electrodes.
The first application was melting metal in electric arc furnaces, which represented a big change from hydrocarbon fuelled furnaces.
One of the patents using transferred arc in this field was U.S. Pat. No. 5,244,488 Ryoda et al., which for the first time did not use the melted material as one of the electrodes, but instead three electrodes between which the arc process took place. Similar principle was employed in the method described in U.S. Pat. No. 2,979,449 Carbothermic reduction of metal oxides by Sheer, C. et al. which used temperatures up to 10 000 K for vaporizing materials and their subsequent condensation and obtaining pure metal.
Similarly, the implementation method of the plasma reactor according to U.S. Pat. No. 7,727,460 used two electrodes, independent of the processed material, for carrying out the transferred arc that vaporizes the material.
In the fifties the first applications of thermal plasma generators came, in particular plasma cutting, welding and plasma coating by metallic and ceramic layers.
The U.S. Pat. No. 2,868,950 Electric Metal-Arc process and apparatus by Gage, R. M., the U.S. Pat. No. 3,082,314 Plasma arc torch by Arata, Y. et al. and U.S. Pat. No. 4,055,741 Plasma arc torch by Bykhovsky et al. describe plasma vortex generators. Their common drawback is a torch temperature limit to a maximum of about 6000 K.
Acme of the use of plasma generators for heat treatment of materials is the concept of coupled generators/twin plasma torch/, which is described in U.S. Pat. No. 6,744,006 Twin plasma torch apparatus by Johnson, T. P. et al. Its advantage is the electrical independence from processed material. The shortcomings are that its scope of action is limited to a line and the big size of the device generating the electric arc.
The closest to the issue of present patent is the material vaporization by a transferred arc in order to generate micro or nanoparticles.
The article: Application of transferred arcs to the production of nanoparticles by Munz, R J., Addona T., da Cruz, A. C. gives an overview on how to utilise an electric arc in order to produce nanoparticles by evaporating the parent material. In PhD thesis: Experimental and modelling study of the plasma vapour synthesis of ultrafine AlN powders, Mc Gill University, Montreal, 1998.
The described systems share one common feature, which is also their drawback, that is the evaporating material forms the anode consumed, one that carries one of the roots of the transferred arc.
Regarding physics of material vaporization process, the vaporization is handled by with a high-power laser beam (MW to TW) but lasting only on the order of microseconds or up to nanoseconds, exceptionally femtoseconds. These principles are not practically applicable for drilling processes, but they are a good theoretical reference source for theoretical work on the processes of vaporization, agglomeration, condensation, clustering, as well as shielding the energy flow from the transferred arc by evaporated rock.
In the article by N. M. Bulgakov and A. V. Bulgakov. Pulsed laser ablation of solids: Transition from normal vaporization to phase explosion.—Appl. Phys. A, 2001, Vol. 73, p. 199-208 the authors describe rapid, almost explosive vaporization of material under the effect of intense heat flow of a laser beam.
Several application papers to use the lasers for rock disintegration in drilling through geological formations were based on this analytical field of pulse material vaporization.
Using laser vaporization, however, has one major drawback. The laser beam is essentially a point source of heat. To cover the entire surface of the borehole it is necessary to blur the beam, which significantly decreases surface power density (W/m2), or to scan the beam across the surface and thereby decrease the power delivered per unit area of 2 to 3 orders of magnitude. Another drawback is the big size of high-power lasers and the need to bring from the surface through optical conduit large power capacity down to the bottom of the borehole (5-10 km), which means substantial losses or the need to use dozens of lasers in parallel.
Similarly important reference source is the use of electromagnetic millimeter waves for fusing, respectively vaporization of the rock for the purpose of drilling described in article: Annual Report 2009, Millimeter Wave Deep Drilling For Geothermal Energy, Natural Gas and Oil MITEI Seed Fund Program, Paul Woskov and Daniel Cohn, MIT Plasma Science and Fusion Center 167 Albany Street, NW16-110, Cambridge, Mass. 02139.
Another promising process of rock disintegration by direct action of an electric arc is the use of the spallation phenomenon, which is based on the overheating of the surface layers causing greater distension in them than in the layers lying underneath them, thus an increase in tension leading to the flaking of the surface layers. The current state of this technology is described in the paper by Ch. R. Augustine in his PhD thesis (MIT), “Hydrothermal spallation drilling” (2009). The current situation drawback is the use of thermal plasma as “hydrothermal flame” working in the supercritical region. This process is difficult to control with large time constants. Also not all rocks exhibit spalling phenomenon. Drilling disintegration technologies based on spallation cannot be then used and must be supplanted by conventional mechanical drilling.
Rock disintegration by thermal effect using the rock phase weakening by thermal effect and subsequent sudden cooling is the standard way of rock disintegration known for millennia.
The patent U.S. Pat. No. 5,479,994 “Method of electrothermomechanical drilling and device for its implementation” by Soloviev G. N. et al. describes a two-phase technology based on the primary rock drying (dehydration) to a temperature of 750-950 K, the following mechanical treatment and in the third step its heating up to 1800-2 300 K. Its disadvantage is the high energy consumption.
For example, for rock containing quartz the heating is carried out preferably above 850 K. At this temperature a phase change occurs and recrystallization, which leads to the volume expansion of quartz crystals analogous to that of a water to ice phase change, leads to the formation of cracks. (Benoit Gibert, David Mainprice: Effect of crystal preferred orientations on the thermal diffusivity of quartz polycrystalline aggregates at high temperature. Tectonophysics 465 (2009) 150-163). Similarly to the cycles of ice freezing and ice melting, cycling around the phase transition temperature increases the efficiency of the whole process of cracking and thus also the process of weakening the rock in terms of its strength characteristics.
Another known method for increasing the disintegration process efficiency is the use of a thermal shock by intensive cooling of the heated volume of rock.
Electrohydraulic phenomenon, described by L. Yutkin in his 1955 paper (“Yutkin, L. A. (1986). Elektrogidravliceskij efekt. Mashinostrojenie—Leningradskoe otdelenie, Leningrad, ISBN 3806811601 forms the theoretical basis for the use of thermal explosive process that generates the pressure shock waves. Further theoretical basis are publications:                Bluhm, H. et al., “Application of Pulsed HV Discharges to Material Fragmentation and Recycling”, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 7, No. 5 Oct. 2000, 625-636;        Dubovenko, K V. et al., “Underwater electrical discharge characteristics at high values of initial pressure and temperature”, IEEE International Conference on Plasma Science 1998 1998;        Hasebe, T et al., “Focusing of Shock Wave by Underwater Discharge, on Nonlinear Reflection and Focusing Effect”, Zairyo (Journal of the Society of Materials Science, Japan), vol. 45. No. 10 Oct. 15, 1996, 1151-1156.        
Weise, Th. H. G. G. et. al., “Experimental investigations on rock fractioning by replacing explosives with electrically generated pressure pulses”, IEEE International Pulsed Power Conference—Digest of Technical papers vol. 1, 1993) describes the use of a thermal effect within the spark cross-section or an arc in the water, with subsequent heat explosion and further generation of pressure shock wave that fragments, or deforms the material in its vicinity.
Shock waves effects and processes were described in detail by J. von Neumann and R. D. Richtmyer “A method for the numerical calculation of hydrodynamic shock” J. of Appl. Physisc 21, 232-237 (1950).
One of the first patent teaching the use of electric arc for drilling in soil (rock) was the French patent no. 727948 under the name “Procede d′execution de forages” from author J. Mekel et al. The claims and description are in very general and broad form and according to the teaching of the patent they contain principal incorrect formulations which would need substantial inventive effort to be realized as it is claimed.
The particular patent's claims concern:    1) The one electrode and the rock as two poles for the arc is not realistic in real rocks because of their very low conductivity which does not allow to ignite the electric arc (the authors admit the failure in the description).    2) The two electrodes concept looks as usable and realistic, but it has substantial, for the intended function serious drawbacks which need intensive inventive activities which are not documented in this referenced patent.            a) The electrical arc has its roots on the nearest points of two electrodes if not under influence of external forces. Patent teaches only magnetic forces pushing in the “down” direction which lets the outer parts of the electrodes in “shadow” of the arc and the rock on the outer side is not melted, is colder with the consequences that the pair of electrodes would not be able to penetrate because of hard or at least high viscosity “cold” melt.        b) The referenced patent teaches the application of strong “violent” stream of gas to the bottom of the drilling, without mentioning any directional control of said stream or its initial direction and interaction with the melted rock and the respective violent cooling effect. The application of strong gas stream is used for flushing the bottom not for creation of continuous plasma flow.        c) The referenced patent teaches to maintain high pressure to the bottom of the drilled wall to press the melted rock to the porous surrounding rock. This concept does not work to remove the melted rock because either the rock is not porous enough and because of cooling effect of the rock does not allow to penetrate the melt deep enough and to place even fraction of the melted rock. The higher pressure as is claimed does help to solve the problem.            3) As shown in previous the referenced patent was too broad and generally foimulated and the claims as we described are not realizable without deep and several inventive steps.
The patent by Clark Malcom, US2308860 “Means of drilling rock, concrete and like” describes a rock drill, comprising a high frequency discharge circuit including a high voltage transformer and a grid control mercury vapour discharge tube, one side of high voltage condenser being connected to ground, the other side of condenser being connected through the discharge tube; a rock drilling arc electrode connected in series with said grid control mercury vapour tube and with the grounded surface to be drilled; an insulated spacer placed between the end of rock drilling arc electrode and the surface to be drilled and power and timing means whereby high voltage condenser may be charged and discharged through mercury vapour tube, thereby producing an arc between said rock drilling electrode and the surface to be drilled.
The solution has following drawbacks:    1) The substance of the concept is the short electrical discharge (spark) and not continuous electrical arc as in our patent    2) The short intensive process is based on intensive heating of water present in the material to the high temperature which causes violent expansion of the water steam in the material and explosive destruction of material and not thermal transformation above the discharge channel.    3) The explosive process produces cuttings of different sizes. The sizes of cutting are not under control.
The further work on this concept was performed at University of Omsk, University of Strathclyde, Dresden University. The discharge of high voltage (150-200KV) was experimentally used at University of Trondheim as electrical discharge assisted to classical mechanical drilling.
The patent WO 2011/037546 by authors Kocis Ivan at al.: “Method of disintegrating materials and device for performing the method” discloses the disintegration of material by high energy water jet and not heat. The high energy water jet is produced by expansion of water by high energy electrical discharge in a pressure chamber filled with water.
The paper Low-Temperature Sintering of Indium Tin Oxide Thin Film Using Split Gliding Arc Plasma from Yukikazu Ito at al., published in Japanese Journal of Applied Physics, Vol. 47, No. 8, 2008, pp. 6956-6959, describes the classical plasma torch configuration where the internal electrical arc between two electrodes in comparison with the proposed spiral gliding arc which allows to heat larger cross section of linearly flowing gas (without nozzle) and then the heated gas is flowing intensively to the surface material. The arc is not near to the material and only flowing heated mediator gas is in the contact with the processed material.